1. Field of the Invention
The present invention is directed to variable camber airfoils wherein the upper surface of the airfoil remains continuously smooth aerodynamically with no abrupt changes in curvature during the airfoil contour change in order to eliminate the high speed drag associated with simple hinged devices that produce an abrupt change in contour and require a space or gap in the contour of the upper skin surface. Although it may be desirable that the upper skin of the airfoil remain unbroken, an overlapping or sliding skin surface could be employed for certain embodiments hereinafter described. Through surface flexing applied to both the leading and trailing edges, the airfoil contour is varied to obtain optimum aerodynamic cruise performance and maneuverability of high performance aircraft such as military fighters and interceptors.
2. Description of the Prior Art
There have been many devices for changing the camber of an airfoil to improve the low speed handling characteristics of an aircraft; however, most of these devices would not be operated at the high speed end of the aircraft performance spectrum.
Leading edge flap or slat devices have been employed for changing the camber of an airfoil, but these devices are primarily employed during low speed flight, such as take-off and landing. Because of the high airloads and structural forces imposed, they are not practical at high speed flight. If they were structurally capable of use at high speeds they would have the disadvantage of producing an excessive amount of drag. Further, the airfoil contours that would be produced by a conventional leading edge flap or slat system would be dependent upon the fixed wing leading edge shape, and therefore, the resulting airfoil contour would be less than ideal. Also, these conventional devices generally produce a break in the continuity of the airfoil contour or the airfoil envelope which produces excessive drag at high speeds. Hinged leading and trailing edges have also been employed, as shown in U.S. Pat. No. 2,650,047, issued Aug. 25, 1953 to Charhart et. al.; and U.S. Pat. No. 2,749,060, issued June 5, 1956 to Brady et. al.; to provide a variation in the envelope contour of an airfoil. However, the external contour of the airfoil is generally not smooth nor streamlined and a sharp break in the contour continuity in generally produced at the hinge line. If the upper flexible skin panel is pulled down without a programmed torsional twist, most of the curvature will take place at one location; e.g., when a rod is bent by gripping one end in a vice and applying a bending force on the other end, all of the curvature takes place right at the vice and the rest of the rod remains relatively straight. This produces a poor aerodynamic surface much like a hinged rigid panel. In a sense, it produces two surfaces at an angle to one another instead of a smoothly contoured surface, which makes a difference in aerodynamic performance. This results in high drag losses that are desirable at cruise flight speeds of present day jet aircraft. It is necessary to program the leading or trailing edge structure, in order to cause the upper flexible wing skin panel to deflect in the desired manner so as to produce the predetermined curvature that will meet the aerodynamically desired contour or camber of an airfoil section.
Crank or bent horn type actuating devices have also been employed to produce the camber variation in an airfoil; however, their lack of stiffness and strength are a problem with this design due to the complex angular, rotational, transverse and telescoping motion, that is required in each of the crank joints. These joints are necessarily large but the supporting crank or bent horn is generally small in cross-section and therefore not capable of carrying the high bending loads that would be imposed during cruise flight speeds of jet aircraft, as is contemplated with the present invention.
In general, most airfoil cambering devices are limited in their capability to provide a predetermined aerodynamic shape. Further, very few prior art devices have practical mechanical designs which lend themselves to functional hardware. With the apparatus of this invention, a mechanical approach to the variable camber airfoil scheme has been disclosed which will satisfy both the aerodynamic and the practical mechanical construction requirements.